The side transaction currently defined in OpObserverImpl::onTransactionPrepare() makes it a challenge to extract some operations that are not directly related to writing the oplog entry. It may be worthwhile to look into defining the boundaries of the side transaction outside the OpObserverImpl method as an intermediate step to extracting some of these operations into a new OpObserver. For onTransactionPrepare, in production code, there is only one caller in TransactionParticipant, which makes this refactoring somewhat straightforward.